It is known that in gas discharge lasers, e.g., utilizing fluorine in the laser gas, e.g., KrF, ArF and F2 gas discharge lasers, there is a great propensity for the production of debris, e.g., in the form of metal fluorides, e.g., due to the interaction of fluorine with metallic components within the laser gas discharge chamber. This can occur particularly during gas discharge, and e.g., with metal materials in the electrodes between which the electric discharge occurs to cause the gas discharge, a chemical and electrical phenomenon that generates radiation. Such gas discharge lasers may be used particularly at or about a selected desired center-wavelength, e.g., for KrF gas discharge lasers at about 248 nm and for ArF gas discharge lasers at about 193 nm. This debris can, over time, plate out on such things a optical components of the laser chamber, e.g., chamber windows, which can cause reduced output power for a number of reasons, e.g., undesired reflection of laser light off of the optic and/or blockage of transmission of laser light through the optic. This can cause, e.g., the need to operate the laser at, e.g., an undesired elevated discharge voltage, e.g., resulting in reduction in laser chamber lifetime. In addition, under some conditions depending on fluence levels and wavelength, among other things, the plated debris can cause, e.g., localized high absorption on an optical element, resulting in earlier than normal failure of the optical element under, e.g., DUV light at high fluence. More importantly, however, dust entrained in the flowing gas can cause, e.g., scatter loss. This phenomena akin to “white-out” on a weather context, can cause the photons generated in a gas discharge between the electrodes of the gas discharge laser to so scatter that they do not reach the mirrors in the laser resonance cavity in sufficient quantities to cause adequate lasing in the excited gas medium during the discharge. This can be significant enough when the dust content is high enough, that no lasing occurs at all in a given pulse or pulses. This phenomenon increases in frequency and likelihood as the dust accumulates in the chamber over the live of the chamber, e.g., measured in billions of shots, and eventually can lead to, or at least be a significant contributor to, what is referred to in the industry as old age syndrome (“OAS”), the onset of which generally requires chamber replacement to maintain, e.g., the required output laser pulse energy (dose), and may also be impacted by such other requirements as pulse to pulse parameter stability requirements being engendered by increasingly demanding requirements, e.g., from lithography tool makers.
It is known in the art of gas discharge laser systems to provide for a debris/dust trap external to the laser gas discharge chamber, with input and output ports from the chamber and returning to the chamber for chamber gas to flow out of the chamber, through the debris trap, and back into the chamber. For example, applicants' assignee has sold gas discharge laser systems with a so-called metal fluoride trap (“MFT”) having a trap inlet and a trap outlet, e.g., near an output window for generated laser light, to flush the area of that window with cleaned gas, as shown, e.g., in U.S. Pat. No. 5,018,161, entitled COMPACT EXCIMER LASER, issued to Akins et al. on May 21, 1991, e.g., as also shown in e.g., the 7000 series and XLA series lasers. Such an external trap may be electrostatic, requiring extra cost and power consumption added to the economics of utilizing such laser systems. Also U.S. Pat. No. 5,373,523, entitled EXCIMER LASER APARATUS, issued to Fujimoto on Dec. 13, 1994 shows an external dust trap on the side of a laser gas discharge chamber. U.S. Pat. No. 6,570,899, entitled GAS LASER DEVICE, issued to Yabu et al. on May 27, 2003, based upon an application Ser. No. 09/648,630, filed on Aug. 28, 2000, illustrates another form of external debris trap.
These types of external traps are also bulky, and tend to fill and become clogged and require replacement, or potentially allow undetected operation over, e.g., several billion laser output pulses (“shots”) of operation with “dirty” gas wherein unwanted OAS events, e.g., zero or low pulse energy lasing are occurring. In addition, they may not be capable of removing debris from the gas circulating within the laser gas discharge chamber fast enough, e.g., at elevated repetition rates of 4K and above, and especially at, e.g., the 6K and 8K and above levels, to prevent detrimental effects on the discharge due to debris presence in the gas between the electrodes at the time of discharge, which, e.g., is variable from discharge to discharge with resultant detrimental effects on such things as bandwidth and wavelength stability, beam shape and spatial coherence stability, etc.
As laser light pulse output repetition rates have increased, along with tighter and tighter controls required on such things as center wavelength, bandwidth and dose and the stability of such characteristics of the laser output light pulses have become necessary for keeping up with the demands of, e.g., integrated circuit lithography light sources, it has become even more important to effectively, efficiently and quickly remove debris, e.g., metal fluoride dust and the like from the circulating gas.
Debris removal from the gas, e.g., between discharges in the gas discharge chamber between the electrodes can, e.g., require very high fan motor speeds that both add temperature to the chamber gas and vibrations that can interfere with meeting laser output light parameter requirements and/or interfere with maintaining stability over time and over different duty cycles and over different output light pulse repetition rates. More debris in the gas can increase rates of deposition of the debris on optical elements, e.g., chamber windows contributing to reductions in performance and/or failures of the optical elements requiring more frequent replacements that are desirable. For these reasons there is a need for an improved debris removal system and method for very high repetition rate narrow band gas discharge lasers. According to aspects of an embodiment of the present invention applicants have proposed additional, low cost, easily implemented and very reliable means for debris removal from the gas circulating within the gas circulation flow path in the laser gas discharge chamber. This also, e.g., extends the life of the MFT, whose function is mostly to maintain a supply of cleaned gas to the chamber window regions.
It is also known in the art of gas discharge laser light sources to utilize preionization of the gas discharge region between gas discharge electrodes that produce the chemical and electrical changes in the gas between the electrodes. Each discharge of electrical energy between the electrodes causes laser light emission and/or amplification, e.g., in an oscillator resonance cavity or an amplification chamber, e.g., amplifying a narrow banded beam output from an oscillator chamber, e.g., in a master oscillator, power amplifier (“MOPA”) configuration. Preionization may be done, e.g., in lasers sold by applicants' assignee with one or more preionization tubes positioned near the gas discharge region. The preionization tubes emit, via, e.g., a corona discharge UV and X-ray radiation which creates electrons via photoionization in the gas between the electrodes assisting the onset of the electric discharge in the gas between the electrodes. Applicants have determined that photoionization in the gas discharge region is less than ideal because most of the electrons are formed in the region of the preionization tube(s) and not in the gas discharge region. Such spatial nonuniformity of the electron distribution is believed by applicants to contribute to adverse effects on energy stability, especially early in a burst of laser output pulses. Applicants, therefore, according to aspects of an embodiment of the present invention propose certain methods and apparatus for improvement of preionization. Applicants propose a number of other improvements for the preionization.
In addition, it is known that acoustic effects in the laser can interfere with proper formation of the discharge in the discharge region, e.g., uniformity in the horizontal or vertical axes of the discharge, which can be caused by a variety of sources of acoustic wavefronts produced in and transmitted through the gas discharge chamber, including, e.g., from the gas circulation fan. Applicants herein propose ways to mitigate or eliminate most of such harmful effects, e.g., on the shape of the discharge.
Another problem facing the operation of gas discharge lasers, particularly as the requirements, e.g., for lithography light sources call for ever narrower bandwidths, bandwidth stability and center wavelength stability shot to shot, power (dose) stability shot to shot or at least over a plurality of shots on average, e.g., within a burst of shots, is in chamber acoustic effects on the repeatability shot to shot. Such requirements for repeatability may, e.g., require essentially exactly the same gas discharge conditions. In addition to the variability of the gas debris content mentioned above there are other possible sources of variability, e.g. two principal sources of these acoustic variations in the gas discharge chamber, shock waves generated from the spinning of and, to a degree, vibrations within the gas circulating fan and acoustic waves created by a prior discharge reflecting back to the discharge in time with a subsequent discharge, and, usually also aligned to the longitudinal axis of the discharge so as to substantially effect the entire length of the subsequent discharge.
According to aspects of an embodiment of the present invention applicants have proposed certain methods and apparatus for the mitigation of these detrimental effects on high repetition rate (e.g., 4 KHz+), high power (e.g., 40 W+), narrow banded, e.g., <about 0.25 pm bandwidth at full width half max (“FWHM”) for ArF and <1.2 pm E95% for ArF and less than about 0.35 pm FWHM and 1.5 pm E95% for KrF, laser light sources. Along with the above are requirements, e.g., for tighter dose stability requirements, e.g., ±about 0.3% for dense lines and less than that for isolated lines, wavelength stability, e.g., ±0.1 pm 3σ, and bandwidth stability, e.g., about ±0.05 pm FWHM 3σ, all of which will become even more stringent requirements as feature sizes (“critical dimensions” “CDs”) continue to decrease with resulting decreases in k1, along with increasing throughput and therefore dose requirements.